Micro-Electro-Mechanical Systems (MEMS), Micro-Opto-Electro-Mechanical Systems (MOEMS), and micromachining technologies are being actively developed by many organizations for implementing devices and systems for a wide diversity of applications in many products and industries. These technologies offer many distinct benefits including: the ability to make complex mechanical elements and systems on a small dimensional scale; the ability to integrate electronics or photonics with micromechanical elements to embody integrated electro-mechanical, opto-mechanical, or electro-opto-mechanical systems; higher levels of reliability; smaller size and weight; higher levels of functionality; and lower cost through batch fabrication techniques. Most MEMS and micromachined devices and systems that have been reported in the research literature as well as those that have been successfully commercialized are made using either bulk or surface micromachining techniques. Furthermore, most have used silicon in single crystal form or deposited polycrystalline silicon as the material from which the devices and systems have been made.
Bulk micromachining is a widely used technique for realizing MEMS and micromechanical components and systems. Using this technique, the wafer or substrate itself is selectively shaped and formed using selective etching techniques so as to implement micromechanical components and systems. Most often silicon is the preferred substrate material used in this technique, but other substrates have been used as well including glass, ceramics, gallium arsenide, etc. Bulk micromachining can be performed using wet etchants or dry plasma etch processes, but most of the more established processes use a wet etchant such as Potassium Hydroxide (KOH) that exploits the anisotrophic nature of etching of silicon crystal in certain crystallographic directions. Commonly, a masking layer (e.g., silicon dioxide, silicon nitride, etc.) or specially modified layer (e.g., heavy boron doping) is used to resist the etching of the silicon substrate at certain locations on the surface of the substrate and provides another means, beyond the crystallographic dependency of the etchant, to form the desired shapes of microstructures. A common bulk micromachining technique is to deposit a thin-film layer that is resistant to the etchant solution onto the surface of a silicon substrate and then to pattern and etch the masking layer so as to selectively expose certain areas of the silicon substrate surface. The silicon substrate is then immersed into a suitable anisotropic etchant and the exposed areas of the silicon substrate are selectively etched. The etchant will remove the silicon from the exposed regions and if the etchant is of the anisotropic variety, the etching will proceed along certain crystal planes at a rapid rate and other crystal planes at a much slower etch rate. The resultant etch profile of the crystalline substrate will display a faceting pattern due to the crystallographic orientation etch dependence.
An etch stop layer is frequently employed in bulk micromachining to enable another degree of freedom in making microstructures of the desired shape or enabling better control of the etching process. The etch stop layer is essentially a layer of suitably modified silicon or a layer of different material that is resistant to the anisotropic etchant. In one method to create an etch stop layer, heavy doses of Boron are introduced into the silicon lattice, usually through a thermal diffusion process. When the etchant reaches the etch stop where the Boron concentration is above a certain level, the etching process essentially terminates. The benefit of the etch stop is that it provides better control of the etching process as well as the ability to make more sophisticated microstructures. Frequently, both a masking layer and an etch stop layer are used in bulk micromachining processes. Bulk micromachining is a very commercially successful fabrication method for microdevices as demonstrated by the fact that it is the preferred fabrication method to realize silicon pressure sensors, which is one of the largest single commercial markets for micromechanical and MEMS devices. Bulk micromachining is one of the oldest and most mature of the micromachining technologies.
Although bulk micromachining is very mature and well established, for many applications it suffers from several disadvantages. Perhaps one of the most important disadvantages of bulk micromachining is that the aspect ratio (defined as the height to width of the microstructure as well as the variation of the unit height to unit width of the critical features, such as the sidewalls, of microstructure) is severely limited. This is principally due to the crystallographic orientation of the silicon material, which limits the sidewall angle to far less than vertical for the most commonly available orientation of silicon wafers, namely the orientation. Although there are methods of anisotropic etching of oriented wafers that will result in near vertical sidewalls and consequently high aspect ratio microstructures, these types of wafers are extremely expensive and the bottom of the etch trench will expose higher order planes that give a faceted trench bottom, which is very undesirable. Another problem with bulk micromachining is that the ability to achieve precision tolerances of devices or microstructures using this fabrication method is quite limited. Although the etching is very slow along certain crystal planes, it is not zero and therefore there is some undercutting of the masking features. Additionally, although bulk micromachining has been a commercially successful fabrication technology, its use is limited to higher margin devices since it is a comparatively expensive method of manufacturing micromechanical and MEMS devices, particularly when compared to the present invention.
Another popular method of fabricating MEMS and micromechanical devices and systems is surface micromachining. In surface micromachining technology, micromechanical devices and systems are fabricated using thin-film layers of materials, which are deposited and etched on the surface of a silicon wafer. Typically, these thin film materials are directly deposited on a substrate using a widely available deposition technology such as Low-Pressure Chemical Vapor Deposition (LPCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), or similar methods including nearly all techniques of chemical and physical deposition. Subsequently, the deposited thin films are suitably patterned so as to form microstructures and micromechanical elements.
Generally, two or more thin-films material types are used in surface micromachining with one of the thin-films acting as a mechanical standoff layer and the other thin-film material acting as a mechanically functional or structural layer. There have been many methods reported in the literature for performing a surface micromachining process with the most widely used method being polycrystalline silicon (also known as polysilicon surface micromachining in the art). In polysilicon micromachining, a layer of sacrificial material, commonly a glass that can be deposited using LPCVD or PECVD such as Phospho-Silicate Glass (PSG) or Low-Temperature Oxide (LTO), is first deposited and subsequently patterned and etched. The patterning and etching of the sacrificial layer is very important in that it defines where the structural or mechanical layer will be directly attached to the substrate. A polysilicon layer is then deposited and then patterned and etched so as to define the shape of the structural or mechanical layer. Frequently, the polysilicon is doped with another material such as Boron or Phosphorus, to make the layer electrically conductive. Subsequently, the sacrificial layer is then selectively removed (by selective removal we mean that the sacrificial layer is removed leaving the structural layer unaltered) using what is termed a “release process” so as to free the structural layer from the constraints of the substrate. Commonly, the sacrificial layer is patterned and etched in a way that the overlaying layer of polysilicon that is subsequently deposited can rest on the substrate in desired locations with the specific purpose of providing a means to anchor the free moving elements made from the released structural material. Consequently, with a suitable design, various movable elements with one or more degrees of freedom can be implemented using surface micromachining fabrication methods.
Like bulk micromachining, surface micromachining has also evolved into a commercially viable method of manufacturing MEMS and micromechanical devices and systems. Surface micromachining does afford several distinct advantages over bulk micromachining including: improved device density, better compatibility of the fabrication process with integrated circuit processes, and the ability to define smaller features and therefore achieve a lower fabrication cost. For example, surface micromachining is the preferred method of manufacturing crash airbag accelerometers for automotive applications wherein a surface micromachined inertial sensor is integrated with sophisticated microelectronics at a relatively low per die cost. However, depending on the intended application of the micromechanical device, surface micromachining frequently has some important disadvantages as well, such as limited mechanical component height and/or mass. For example, if a movable microdevice such as an inertial sensor needs sufficient mass for an intended application but is constrained to a limited lateral area due to cost or processing restrictions, conventional surface micromachining may not be a suitable fabrication process since the device will be made from thin-films and therefore will have very small mass levels. The only way to significantly increase the mass of the micromechanical element in surface micromachining is to make the lateral area of the device extremely large, but this consumes considerable substrate area and therefore is very costly and reduces production yield.
More recently, some new MEMS and micromachining processes have become available wherein a very high aspect ratio and relatively tall or deep microstructure can be readily achieved. One of these processes is called Deep Reactive Ion Etching or DRIE which is similar to bulk micromachining described above, but provides a method for achieving a higher aspect ratio for making microstructures and is performed using a dry plasma etch process. In the DRIE process, the single crystal silicon is etched using a dry plasma etching process wherein the etch chemistry is cycled repeatedly from a polymerization cycle to a highly anisotropic silicon etch cycle and back. During the polymerization cycle the sidewalls are coated with a thin protective polymer layer, while during the etching cycles the sidewalls are not attacked but the trench bottom is preferentially etched. Additionally, a masking material such as a thick photoresist or an oxide layer are commonly used to prevent etching of the silicon surface or other pre-defined features in selected areas. The result is a near vertical sidewall in the silicon substrate (i.e., very high levels of anisotrophy) to define very high-aspect ratio microstructures. The etching processes for DRIE have been modified so as to increase the silicon etch rate to high levels, thereby enabling very deep etches into the silicon material to be performed in reasonable times.
One disadvantage of the DRIE micromachining process is that the etch rate is non-uniform depending on the size of the areas exposed. For example, if a substrate has both very small and very large openings in the masking layer, the etching will typically be considerably faster in the larger open areas and slower in the smaller open areas.
Another disadvantage of DRIE technology is the cost. While this technique solves some of the problems of bulk micromachining, it is a single wafer process that takes considerable time on expensive tooling. For example, a common DRIE process is to etch holes completely through the wafer. But this single process step costs as much as a CMOS microelectronics process consisting of hundreds of individual processing steps. Consequently, the technique is limited to very high margin devices and systems.
Frequently in the art, so-called Silicon-On-Insulator (SOI) wafers are the preferred substrate for use in DRIE etching processes. In SOI wafers, an oxide layer is sandwiched between two single crystal silicon layers: one termed the handle wafer and the other the device layer. The device layer can be controlled to a desired thickness ranging from a couple of microns to many hundreds of microns, and it is this layer that is typically etched using DRIE to make the micromechanical elements. One advantage of DRIE etching performed on the device layer of SOI wafers is that the DRIE etching terminates on the buried oxide layer thereby providing better etch characteristics. Despite the ability to make tall and high aspect ratio microstructures using DRIE, the main disadvantage, which has severely limited its use for manufacturing, is cost. Furthermore, if SOI wafers are used in the process, this increases the cost even more since these are relatively expensive substrates compared to single crystal silicon wafers.
Yet another method for making tall and high-aspect ratio micromechanical elements is a process technology called LIGA, which is a German acronym for Lithographie, Galvanoformung, and Abformung, which translates in English to lithography, galvanoforming, and molding. A key element of the conventional method of performing the LIGA process is the use of X-ray exposure for the lithography, which enables exposure through very thick photosensitive polymer layers as well as extraordinary high aspect ratios and extremely high resolutions. Typically, a relatively thick layer (e.g., hundreds to thousands of microns in thickness) of PMMA is put onto the surface of a substrate (although other photosensitive polymers can be used as well) and a specially configured mask is used to perform the exposure. The PMMA layer is then selectively exposed through a specially configured mask using an X-ray source, such as a synchrotron. The high energy and short wavelength of this radiation source provide exposure of the mask pattern completely through the thick PMMA layer with submicron lateral resolution. The exposed PMMA layer on the substrate is then developed so as to remove the exposed PMMA material and leaves behind the areas of PMMA not exposed to the X-ray radiation. Electroplating into the exposed and developed thick PMMA layer is then performed. Typically a metal such as gold or nickel is plated slightly beyond the height of the PMMA layer. The top surface of the electroplated metal is polished so as to remove the surface roughness. The PMMA layer is then selectively removed leaving the metal plated structure. The metal plated structure can be used as the actual part, but commonly it is used to define a mold in order to replicate the part. Replication is performed by making a reverse pattern mold from the shape of the metal plated part out of a polymer material. A metal is then plated into the polymer mold and subsequently the polymer is removed leaving the copied metal part.
The advantage of the LIGA process is the ability to implement microstructures with heights of hundreds to thousands of microns and near vertical sidewalls with submicron lateral resolution. The principal disadvantage of LIGA technology is the cost. On a relative cost scale, LIGA is usually the most expensive micromachining process, followed by DRIE technology on SOI wafers, and then bulk micromachining, with surface micromachining on the lower end of the cost scale.
Nevertheless, for many MEMS and micromechanical devices, the currently known fabrication technologies to implement high-aspect ratio devices are far too expensive. Consequently, there is an enormous opportunity for fabrication methods whereby high-aspect ratio microstructures that are tall and have sub-micron lateral features are achieved.
Another disadvantage of LIGA technology is that it is not well suited to make movable components. Most all LIGA-fabricated devices reported to date have been microstructures whereby a static microstructure is shaped or formed from the material. For example, various types of fiber-optic alignment structures have been reported which were made using LIGA fabrication methods. Essentially, these devices consisted of precision micromachined grooves that the fiber optic cable was inserted into so as to hold the cable in position. However, there is a tremendous need for a process to fabricate high-aspect micromechanical devices and systems whereby various elements can move with one or more degrees of freedom.